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Statistics of Saturn’s Ring Occultations: Implications for Structure, Dynamics and Origins

Presentation #112.05 in the session Many Planets, More Rings (Oral Presentation)

Published onOct 23, 2023
Statistics of Saturn’s Ring Occultations: Implications for Structure, Dynamics and Origins

The varying geometry of Cassini star occultations by Saturn’s rings constrains both the size and shape of structures that block starlight. C ring. We find similar effective particle radius in the C ring background & ramp: aeff = 2.35m (βCen 077I), 2.65m (αVir 031I), agreeing with aeff = 2.38m Jerousek (2016). No evidence for gaps or ghosts in these 2 regions. Mid A Ring: In the central featureless region, there are no density waves or viscous overstability, and self-gravity wakes dominate the statistical moments. Gaps or ghosts are required to match skewness and kurtosis, especially for low declination occultations. Gaps demonstrate dynamic processes which prevent the rings from achieving uniformity. We require a third, transparent mode in the ring particle spatial distribution, beyond individual particles and clumps: The A Ring structure includes empty alleys parallel to the wakes, as suggested by numerical simulations of Tiscareno (2010) and Salo (2018). UVIS occultations over a range of declination give the best fit for the empty trough width T3 = 14 ± 4m; with vertical thickness 2< H <12m. Rehnberg (2016) fit an exponential gap size distribution with size scale HS ~ 10m; Max observed width = 50-100m. The largest unstable wavelength λT is roughly twice this, giving surface mass density ∑ = 70-140g/cm2 at r=127,000km, somewhat larger than found from nearby density waves dispersion. Density waves trigger aggregation: For βCen 105I, Janus 5:4 density wave, moments from the data are not symmetric about the wave phase = 180°; Structure is different just after compared to just before the density wave crest passes. This lag is consistent with the predictions of the Predator-Prey dynamics model for density waves triggering growth of aggregates. This agrees with Zarah Brown (2016) that gaps are significantly more abundant in the density wave troughs. Excess Variance shows self-gravity wake width W = 18-29m; wavelength S+W ~ 60m; H/W < 0.12, thus vertical thickness H < 4m, consistent with N-body simulations.. Perturbed by passing density waves, self-gravity wakes grow and erode on orbital timescales with a full amplitude of 60%, and a phase lag φ ~45°. Straw & Dust haloes: The collisions or azimuthal instabilities of these wakes may lead to the straw features seen in the Cassini images. Ejecta from self-gravity wake collisions and erosion (Becker (2018) showed similar dust production in analysis of sun occultation by the F ring) form dusty haloes around the density waves. The resonance driving produces both aggregates and subsequently small particles!

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